The general objective of this work is to develop a cheap,easily used system that can be used in the detection, characterization and ultimately treatment of tumors. The specific aims of the project are: 1)Extend a previous in-house study to examine the possibility of using coaxial line probes to characterize the reflections between different tissue structures. 2)Test the optimized design using a simple inhomogeneous dielectric phantom. Optimization and characterization of the probe will be accomplished by the use of a previously developed three dimensional finite difference time domain (3D FDTD) computer program. This powerful technique enables us to calculate the fringing field patterns of various probes without assuming aperture field distributions. After a design has proved successful at a theoretical stage, the probe will be interfaced to a Network Analyzer and tested against a layer dielectric phantom. The response of the probe will then be examined for ease of detecting the various reflection coefficients that result from the interfaces. Preliminary experimental data taken for in-vivo skin characterization suggests the study should commence by optimizing the performance of previously developed probes. However, since the FDTD program is both fast and flexible we anticipate other kinds of applicators will also be examined.